1. Field of the Invention
The present invention pertains to magnetic recording, and particularly to apparatus which record/reproduce an alternating-azimuth recorded track pattern on magnetic tape.
2. Related Art and Other Considerations
Examples of helical scan apparatus (e.g., helical scan tape drives), including examples which have alternating-azimuth track patterns, are described in the following non-exhaustive and exemplary list of United States patents, all of which are incorporated herein by reference in their entirety: U.S. Pat. No. 5,065,261; U.S. Pat. No. 5,068,757; U.S. Pat. No. 5,142,422; U.S. Pat. No. 5,191,491; U.S. Pat. No. 5,535,068; U.S. Pat. No. 5,602,694; U.S. Pat. No. 5,680,269; U.S. Pat. No. 5,689,382; U.S. Pat. No. 5,726,826; U.S. Pat. No. 5,731,921; U.S. Pat. No. 5,734,518; U.S. Pat. No. 5,953,177; U.S. Pat. No. 5,973,875; U.S. Pat. No. 5,978,165; U.S. Pat. No. 6,144,518; and, U.S. Pat. No. 56,288,864.
Alternating-azimuth track patterns have been routinely used in both analog (e.g., VHS, Betamax, etc.) and digital helical tape recording format since the mid 1970s. Examples of digital helical tape recording formats are D-2, DCT, D-3, 850X, and Mammoth™. In all of these alternating-azimuth helical recording formats, the tracks are recorded in a sequentially overlapping fashion in both time and space: a “+” azimuth track, followed by a “−” azimuth track, followed by a “+” azimuth track, followed by a “−” azimuth track, etc.
FIG. 1 shows an example of the Mammoth-2 helical scanner architecture which uses this sequential (prior art) recording method. The four write heads (W1, W2, W3, W4) mounted on the periphery of a rotatable drum or “scanner” are grouped into two local modules of two heads each (a first module with heads W1, W2; a second module with heads W3, W4). The two modules are separated by 180°. Since the recorded track length corresponds to about 180° of the drum's rotation, the signals for the four write heads can be efficiently supplied by only two signal channels. The timing of the signals for the respective write heads W1, W2, W3, and W4 are as shown in FIG. 1A.
FIG. 2 shows a view of the magnetic tape surface for a helical scan tape drive known as Mammoth™-2, and serves to illustrate the “sequential” nature of the time and position relationships of the recorded tracks. In FIG. 2, the W1 head (+ azimuth) is just finishing writing Track 1i. The W2 head (− azimuth) is closely following the W1 head while it writes Track 2i in an overlapping fashion. On the scanner, the vertical height from the bottom of the W1 head to the bottom of the W2 head is adjusted nominally such that the W2 head follows a path on the tape the distance “P” above the path of the W1 head as shown in the FIG. 2. The W3 head (+ azimuth) is just at the start of what will become Track 3i. The vertical height from the bottom of the W1 head to the bottom of the W3 head is adjusted nominally such that the W3 head follows a path on the tape the distance “2P” above the path of the W1 head. The W4 head (− azimuth) will closely follow the W3 head while it writes Track 4i in an overlapping fashion. The vertical height from the bottom of the W3 head to the bottom of the W4 head is adjusted nominally such that the W4 head follows a path the distance “P” above the W3 head as shown in FIG. 2.
Repeating this process (+ azimuth write, followed by − azimuth write, followed by + azimuth write, followed by − azimuth write, etc.) produces a recording which comprises a series of alternating-azimuth tracks, with each track being of nominal width P.
One problem with the approach described above is that it is difficult to hold the positional relationship between the W1/W2 head module and the W3/W4 head module, particularly since these modules are separated by a relatively large physical distance. When there is an error, “Δ”, between the W1/W2 head module and the W3/W4 head module, the recorded track pattern is severely affected as shown in FIG. 3. In this example, the vertical height from the W1 head to the W3 head is such that the W3 head follows a path not 2P, but rather, 2P−Δ from the W1 head path. Consequently, the width of every Track 2 is reduced to P−Δ, and the width of every Track 4 is increased to P+Δ.
What is needed, therefore, and an object of the present invention, is a helical scan drive apparatus and method of operating the same which will transduce tracks of proper width despite a variation or error in separation of the head modules.